Project Ara is the codename given to a project by Google. It aims to build modular smartphones. It envisages a skeletal main frame on which you go on adding or removing different modules at your wish. These modules could be a camera module, a display module and so on and so forth. The parts can be upgraded or removed at owner's wish.
The essential structure of an Ara phone will have modules 'inserted' into a metal endoskeletal frame. You can also remove a model when the phone is running just like we eject the USB drives from the PC.
It is too fantacy world like as of now, but Google is working on this concept to make it a reality. What Google wants is to remove the entry barrier for the component manufacturers by allowing a bunch of developers for the modules rather than the current handful of them.